Simulating Operational Amplifier circuits with pSpice

Besides basic devices (diodes, JFETs, MOSFETs etc) pSpice has a range of
numerical models for more complex parts such as operational amplifiers. We can use
these to confirm some of the results deduced in lectures by analysis and calculation.

The Op-amp chosen for these simulations is the 741. This has been a
popular design for many years, and is available at very low cost from a number of
different manufacturers. The schematic below shows the main features of this device.

The inverting input, non-inverting input and output terminals can be
clearly seen.

Terminals V+ and V- are the positive and negative power supplies to the
chip. Often left out of schematics, these are essential connections!

Terminals OS1 and OS2 are fine adjustments to ensure the output is
exactly at 0v when the input is at 0v. It's often possible to ignore these in
non-critical applications, like audio.

The schematic below shows Design Example 18.1. The preferred values
for the capacitors are shown. The output terminal is loaded with a resistor R3 of
15k. Since the output resistance of the circuit is reduced by negative feedback,
the presence of this load does not cause any noticeable reduction in output.

Shown below is the frequency response. The axes as logarithmic:
frequency is shown in convenient logarithmic decades, while gain is shown in decibels. The
mid-band gain is expected to be x -10 (or 20 dB) and the -3 dB points (where the
gain falls to 70.7% of the gain in mid-band) should be at 100 Hz
and 5 kHz. This can be confirmed from the graph. However, notice how the
actual gain does not quite attain the expected value of 20dB in mid-band.

Just in case this is due to our use of 'preferred' values rather than the
precise calculated values, the corrected circuit shown below in Fig. 2a is also simulated
- results in Fig. 2b. It can be seen that the gain does not reach 20dB in this case,
either.

The reason that the design gain of 20 dB is not reached in mid band is
that the upper and lower cut-off points are rather too close together. In the middle
range of frequencies the 159nF capacitor is still inserting a noticeable (though
small) reactance in series with the input resistance. The parallel capacitor of 318
pF has sufficient admittance to affect the feedback current. We can confirm this by
plotting the response with a larger value for C1 - say, 1590 nF instead of 159 nf - an
increase by a factor 10. This is shown below.

Increasing C1 to 1590 nF pushes the lower -3dB point down by a factor 10,
and there is an increased separation between the lower and upper cut-off (-3dB)
frequencies. We can now see that the gain achieved is now much closer to 20 dB over
a substantial frequency range (from about 150 Hz to 300 Hz).

We can conclude from this that for there to be an identifiable 'mid-band'
range of frequencies our design must have a sufficiently large separation between the lower and
upper -3dB points.